Electric double layer capacitors have now come to be used not only in such major applications as power smoothing and noise absorption, for which the conventional capacitors (also referred to as condenser) are used, but also used as a supplement or alternative to a secondary battery and a power source for memory backup in a personal computer because the electric double layer capacitors have a large capacity comparable to NiCd batteries, nickel hydride batteries, and lithium ion batteries.
Although the conventional secondary battery has a large capacity, the lifetime thereof is relatively short and it is difficult to achieve quick charging and discharging thereof. In contrast thereto, the electric double layer capacitor has, in addition to the relatively large capacity, such good characteristics as a capability of quick charging and discharging and a long lifetime, which are prime advantages inherent in the capacitor.
The electric double layer capacitor generally comprises positive and negative electrodes, an electrolyte, a separator, and an electricity collecting plate, and the separator is used for the purpose of enhancing circulation of the electrolyte while avoiding a contact between the positive and negative electrodes. The separator is desired to have a small thickness since increase of the thickness of the separator results in increase of the path between the positive and negative electrodes, resulting in increase of the internal resistance. For this purpose, fibers forming the separator are desired to be extremely fine so as to reduce the thickness of the separator.
As an electrolyte in the capacitors, an aqueous electrolyte (e.g., an aqueous solution of sulfuric acid) or an organic electrolyte (e.g., a solution containing tetraethyl ammonium/tetrafluoroborate dissolved with propylene carbonate) is used, and the organic electrolyte has recently been highlighted because the electric decomposition voltage thereof is higher than that of water and enough to increase the energy density. With respect to the organic electrolyte, however, it is necessary to remove water as much as possible since water constitutes an impurity and tends to lower the capacitor performance. For this reason, the sufficient removal of water from the separator is generally carried out by drying the separator at a high temperature under a vacuum atmosphere. Therefore, the separator used in the capacitor of a kind using the organic electrolyte is desired to have a high heat resistance enough to withstand against the drying at high temperatures.
As for the separator used in the electric double layer capacitor, there have been known a separator comprising cellulose-series fibers (see the Patent Documents 1 to 3 listed below), a separator including a layer of an extremely fine polyacrylonitrile fiber web, the separator having a thickness of not greater than 25 μm in total (see the Patent Document 4 listed below), a separator for electric double layer capacitors comprising a nonwoven or woven fabric containing, as the subject fiber, sulfonated polyolefinic fibers to impart a hydrophilic property (see the Patent Document 5 listed below).
However, the separator prepared from the cellulose-series fibers has been found having insufficient heat resistance as it is easily discolored into a brown color when heated to a temperature of not lower than 150° C., and therefore the separator lowers the physical properties.
Moreover, the separator including the layer of the extremely fine polyacrylonitrile fiber web has been also found having an insufficient heat resistance because thermal shrinkage is apt to occur when it is heated.
Furthermore, even in the separator using the nonwoven or woven fabric containing sulfonated polyolefinic fibers, due to its insufficient heat resistance, the separator has been found not being suited for use in the electric double layer capacitor of a kind utilizing the organic electrolyte, in which water in the capacitor is generally required to be completely evaporated through drying at high temperatures.
In addition, there have been well-known separators for use in the electric double layer capacitor (see the Patent Documents 6 to 10 listed below); these separators are prepared from a wet-laid nonwoven fabric obtained by the wet paper making process using a sheet making material containing heat resistant fibrillated fibers and/or heat resistant short fibers.
Those separators have an improved heat resistance because they are prepared from the heat resistant fibrillated fibers and/or short fibers. However, of those prior art technologies, the separator of the wet-laid fabric comprising the refined fibrillated fibers and/or the short fibers has a small fineness and a too low porosity due to the wet paper making process, and, therefore, not only does the electrolyte pass insufficiently, but the internal resistance tends to increase, when such separator is used in the capacitor. As a result, the capacitor sometimes fails to perform smooth and stable charging and discharging. On the other hand, the separator obtained by the wet paper making process and comprising conventional short fibers having an average fiber diameter of micrometers or approximating thereto has been found having too large pores and, accordingly, not only does a substantial amount of electric current tend to leak, but also short-circuiting tends to occur because when such separator is used in a capacitor, electrode materials detached from the positive and/or negative electrodes may pass through the separator.    Patent Document 1: Japanese Laid-open Patent Publication No. 9-129509    Patent Document 2: Japanese Laid-open Patent Publication No. 11-168033    Patent Document 3: Japanese Laid-open Patent Publication No. 2000-40641    Patent Document 4: International Publication Pamphlet No. WO2006/049151    Patent Document 5: Japanese Laid-open Patent Publication No. 2001-68380    Patent Document 6: Japanese Laid-open Patent Publication No. 2001-185455    Patent Document 7: Japanese Laid-open Patent Publication No. 2002-151358    Patent Document 8: Japanese Laid-open Patent Publication No. 2005-159283    Patent Document 9: Japanese Laid-open Patent Publication No. 2006-135243    Patent Document 10: Japanese Laid-open Patent Publication No. 2007-150122